dotnuet nuget why uses current directory when provided with one argument

[zivkan]

Bug

Fixes: NuGet/Home#13553

Description

Big thanks to @baronfel for the advice on how to get this to work.

Change the path argument to use a custom parser, where we can tell System.CommandLine if we want to take zero or one tokens from the parsed argument list. When we take zero, we use the current directory as a default.

I also made why's command line parsing configuration unit testable, so we can easily and exhaustively test all scenarios we care about, and it's still super fast (compared to invoking dotnet nuget why in dotnet.integration.tests, waiting for the process to end, and checking the exit code).

Also moved some code and namespaces around, to make things more intuitive/how other projects organise files and namespaces.
 

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.
  • update dotnet nuget why docs for optional path argument Home#13709

[nkolev92]

do we need this change?

I still like it when we had this under a Choose/When condition since it was just easy to work with, but that's not the topic of this PR.

[zivkan]

No, we don't need that change. I made such an effort to avoid commiting this file (and uncommenting the related lines in program.cs) on each commit, but all it took was one slip-up ☹️ Then, instead of "reverting" the file, I tried using the IDE to comment out the same lines, but it didn't do it the same way as the code before.

I also prefer the choose-when for debug mode. I don't know what problem was trying to be solved by having debug and release builds work the same, but in my experience working on this bug, it makes development harder. The debug-only code was just easier to work with and had no impact on the optimized binaries that we ship to customers.